Fixed-wing aircraft, such as airplanes, are capable of flight using wings that generate lift responsive to the forward airspeed of the aircraft, which is generated by thrust from one or more jet engines or propellers. The wings generally have an airfoil cross section that deflects air downward as the aircraft moves forward, generating the lift force to support the aircraft in flight. Fixed-wing aircraft, however, typically require a runway that is hundreds or thousands of feet long for takeoff and landing.
Unlike fixed-wing aircraft, vertical takeoff and landing (VTOL) aircraft do not require runways. Instead, VTOL aircraft are capable of taking off, hovering and landing vertically. One example of a VTOL aircraft is a helicopter which is a rotorcraft having one or more rotors that provide lift and thrust to the aircraft. The rotors not only enable hovering and vertical takeoff and landing, but also enable forward, backward and lateral flight. These attributes make helicopters highly versatile for use in congested, isolated or remote areas. Helicopters, however, typically lack the forward airspeed of fixed-wing aircraft due to the phenomena of retreating blade stall and advancing blade compression.
Tiltrotor aircraft attempt to overcome this drawback by including a set of proprotors that can change their plane of rotation based on the operation being performed. Tiltrotor aircraft generate lift and propulsion using proprotors that are typically coupled to nacelles mounted near the ends of a fixed wing. The nacelles rotate relative to the fixed wing such that the proprotors have a generally horizontal plane of rotation in a VTOL flight mode and a generally vertical plane of rotation in a forward flight mode, wherein the fixed wing provides lift and the proprotors provide forward thrust. In this manner, tiltrotor aircraft combine the vertical lift capability of a helicopter with the speed and range of fixed-wing aircraft.
It has been found, however, that tiltrotor aircraft may occupy a large footprint when not in use, such as during storage on an aircraft carrier flight deck. Accordingly, certain tiltrotor aircraft are operable to perform a conversion from flight mode to storage mode, as seen in prior art FIGS. 1A-1D. In FIG. 1A, a tiltrotor aircraft is shown in VTOL flight mode with the nacelles positioned in a generally vertical orientation and with the proprotors operable for rotation in a generally horizontal plane. In FIG. 1B, two of the rotor blades of each proprotor have been folded in the beamwise direction such that all blades are generally parallel to the wing. In FIG. 1C, the nacelles have been rotated approximately ninety degrees relative to the wing to a generally horizontal orientation. In FIG. 1D, the wing has been rotated approximately ninety degrees relative to the fuselage of the tiltrotor aircraft such that the wing is generally parallel with the fuselage. In the illustrated storage mode of the tiltrotor aircraft, its footprint has been minimized. It has been found, however, that storing a tiltrotor aircraft with the rotor blades fully cantilevered to one side of the drive system results in an undesirably large moment being placed on the drive system, which may cause damage to bearings or other components of the drive system. Accordingly, a need has arisen for improved storage modes for tiltrotor aircraft.